Updated: Jun 1
The pelvis is like a bowl that contains most of our organs, and it is influenced by the movements that rhythmically happen between them. That internal "push" from the organs is essential to the development of the pelvis.
In cerebral palsy, due to the weakness of the abdominal wall, the movement of the organs is not always translated into the pelvis. This is also one of the reasons why they are usually constipated.